Wireless networks, particularly radio access networks, are configured to operate within specific frequency ranges. To mitigate interference problems, an operator of a radio access network (RAN) is licensed for its use in specific bandwidth ranges. The license often restricts the Radio Access Technology (RAT) that may be used. For example, older RATs (such as Global System for Mobile Communications (GSM) and General Packet Radio Service (GPRS)) have conventionally been allocated in frequency bandwidths around 1800 MHz. It is increasingly desirable to operate RANs with a different RAT, such as Long Term Evolution (LTE), in the same bandwidth ranges as those in which RANs with such older RATs are operating. The spectrum can be split between the two RATs in a fixed manner in order to avoid significant interference between the two systems. Such a fixed share is not efficient however, since traffic varies significantly over time. There will be times when the load on one RAN is greater than the other and vice versa.
Desirably therefore, spectrum is shared between the two RANs. The proportion of the spectrum used by each RAN can be varied over time. One approach for achieving this is known as Dynamic Spectrum Sharing (DSS), in which a first portion of a bandwidth range is allocated to a first RAN and a second portion, contiguous (adjacent) with the first portion, is allocated to a second RAN. Both portions together form a single continuous bandwidth range. Referring to FIG. 1A, a schematic diagram showing an example spectral allocation according to the known approach of DSS is shown. Here, the first portion of spectrum 10 is allocated to LTE and the second portion of the spectrum 20 is allocated to GSM. These portions can be dynamically adjusted on the basis of the level of traffic being carried by each RAN.
In an alternative approach, a GERAN is allocated one or more specific frequency segments within a bandwidth range, which may be contiguous or non-contiguous. Then, an LTE RAN is allocated all of the remainder of the bandwidth range that is not occupied by the GERAN. As the GERAN allocation is varied, the LTE RAN allocation changes accordingly. This approach can be termed Full Spectrum Sharing (FSS). Referring to FIG. 1B, a schematic diagram showing an example spectral allocation according to the known approach of FSS is shown. Here, the first portion of spectrum 30 is allocated to LTE and the second portion of the spectrum 40 is allocated to GSM. LTE is not scheduled frequencies occupied (used) by GSM traffic 40. It is potentially more efficient, since the LTE RAN may be provided with a full carrier for the entire bandwidth range. In contrast, only a fraction of the bandwidth range could be used by the LTE RAN under DSS.
Techniques based on DSS, FSS or both are described in European Patent publication No. EP2203011, having common ownership with this invention, and in International (PCT) Patent Publication No. WO-2010/091713. Allocations to each RAN under FSS can be made on the basis of each network's load, as for DSS. This allows load balancing. Nevertheless, improvements to load balancing in FSS remain a challenge.